Enabling the TPA6404-Q1 for Good EMI Performance

Hello. In this presentation, we will be discussing how to enable the TPA6404-Q1 Class-D Automotive Audio Amplifier for good EMI performance. In today's automotive audio application, space has become a premium. We will learn how to meet the space requirements and still meet automotive CISPR25 EMC requirements using a TPA6404-Q1 and its surrounding components.
You'll learn how to help a customer design their automotive audio amplifier system in respect to the Electro-Magnetic interference. We will learn about the advantages of the TPA6404-Q1, its external components, and PCB layout. First, we'll be looking at the benefits of the TPA6404-Q1 for meeting EMC, some refresher basics of EMI, the external components from a schematic point of view and the material they're made out of, and the PCB layout with potential EMC problems and best practices.
The benefits to the TPA6404-Q1 is that the PWM switching frequency is 2.1 megahertz, where a lower value and smaller size inductors are used, allowing for compact PCB layout. It also allows for lower value capacitors. But with 2.1 megahertz, there are also faster edges on that PWM signal, and the rise and fall current is faster in the LC filter.
The package has a flow through design, which has a high current, or noisy pins, on one side of the device and the lower current, or quiet pins, on the other side. Be careful. The flow through design can also create a dipole antenna.
And now for a little bit of an EMI refresher. Electromagnetic Emissions are signals that can both be periodic and non-periodic and have dv/dt and di/dt components. There are four emission paths. The electric field, which is capacitive coupling through dv/dt, and there needs to be two conductive surfaces for this type of coupling. We have conducted, and this is noise that is coupled by conductors that have the same shared resistance paths, not necessarily has the same potential, but have conductive connections.
There is magnetic field. This is inductive coupling through di/dt, and there needs to be two inductive loops for this type of coupling. Then there's electromagnetic, and this is far field or radiative, and there needs to be two antennas for this type of coupling.
Capacitance coupling through the PCB is the most probable point of coupling for electric fields. As you can see, there needs to be two conductive plates, typically on each side of the PC board or an internal layer. As you can see from the dimension d, it's dependent on that. So the smaller d is, the larger the capacitance. In today's 10-layer boards, d can be very, very small. And now this can be a very good source of EMI infecting other parts of your circuit.
In this slide, we'll discuss magnetic field. As you can see in the diagram, that inductance must have a complete loop. And this loop has a radius and a wire diameter. In the equation, you can see that the inductance is directly proportional to the radius, or the area inside the loop.
In the continuation of the magnetic field EMI, you also have mutual inductance. The mutual inductance is where the magnetic field or the flux generated by a loop of wire induces current into an adjacent loop of wire. In this case, Loop 1 is the aggressor and creates a flux due to the current in the loop. Loop 2 is the victim, because some of the flux travels through the loop and induces a current in Loop 2. One can think of this as an air-core transformer.
Conducted EMI, this is where two conductors share a path or are resistive path. So what happens when Loop 1 and Loop 2 partially share a common conductor? This is one method of conducting EMI where the aggressor is directly connected to the victim. And what happens if the red lines have an equal magnitude in Loop 2 and flow to the other direction? Well, we will discuss this stuff as we examine the TPA6404-Q1 system design in detail.
So now, let's look at the application schematic for the TPA6404-Q1. We will be breaking this down into four areas of concern-- the audio input, the power input, PVDD decoupling, and the output LC filter. This is the TPA6404-Q1 EVM PCB.
This is the top view. Under the red heat sink is the TPA6404-Q1. On the left side is the RCA low level audio inputs, and then on the right side are the speaker outputs. And for reference, we also included the bottom view of the TPA6404 EVM PCB. As you can see, we also have some inductors on the bottom here and some power supply parts. In this case, on the right hand side is the RCA inputs, and on the left hand side are the speaker outputs.
This is the audio input block. The source for the audio is on the left hand side of the circuit, and it drives through the circuit to the input side of the TPA6404. There are two 499 ohm resistors in series, and between those resistors is 1,000 picofarad to ground.
The audio travels from left to right, but the EMI from the TPA part goes from right to left. The 499 ohm resistor 1,000 picofarad capacitor create a simple RC low pass filter. It is needed to keep the high frequency contamination from influencing the preceding stage.
This filter is a two-way filter with two resistors and a cap. This also filters any high frequency from entering the amplifier from the source side. And of course, a PCB layout is important for this circuit.
Let's take a look at the PCB layout at the audio input block. As shown in the upper diagram, and in the lower left hand diagram, we see that the parts must be close to the noise source. In this case, it's the TPA6404. In the lower right hand side diagram, we show that the ground connections must return to the device in a short route. We need this short loop so that there is a very low inductance loop, so there is very little mutual inductance between all the loops of these different devices. Remember, this is also located in a low current sided device, so the noise should be low.
This is the power input filter block schematic. The main power, or DC current, flows from the battery to the TPA6404-Q1 shown by the green arrow. The EMI current actually flows from the TPA6404-Q1, which is the noise source, to the battery. As shown, the LC filter works against the EMI current. And we also show that we use multiple capacitors in parallel with selected values to contour the LC filter response at frequencies near the component residences.
Remember, package sizes have different resonance values, as well as different capacitor values have different resonant values. And different manufacturers have different values of resonance, also. These components' values are highly dependent on the systems design, and not just the TPA6404-Q1.
Let's look at the PCB layout of the power input filter block. The parallel capacitor should be close to the inductor and also the power input pins, which means that the inductor and the power pins should be close together with the capacitors in between. And the ground should be connected to a good ground plane, and the return path to the noise source, and close to the ground connections.
So again, everything should be close. And all the filter parts should be on the same side of the PCB. Remember that the large bulk filter capacitor, to 330 microfarad in this case, is not part of the EMI filter, but this is needed for stabilizing PVDD in the audio band.
This is the PVDD and VBAT block schematic. These are critical decoupling capacitors on the PVDD and VBAT pins. Again, we parallel capacitors with selected values to contour the filtering at frequencies near the component resonances.
Now, let's look at the PCB layout. The capacitor should be as close to the PVDD and VBAT pins as possible, and the lowest value should be the closest to the pin. The lowest value should be closest to the pins so that the highest frequency loop is the smallest, as the loop inductance will be more critical at the higher frequencies as it is at the lower frequencies.
And the ground connection needs to be as close to the 6404-Q1 ground pins as possible, again, to keep that loop small. And this is shown by the four arrows on the blue diagrams at the top. As I always stress, no PVDD planes, the path from the bulk capacitor to the IC should be direct.
Let's look at the output filter block schematic. The LC filter must be designed to allow for good audio performance and to reject the 2.1 megahertz PWM signal. In this case, a good compromise is around 100 kilohertz to allow for wide audio bandwidth and good rejection at 2.1 megahertz. A filter with a slightly higher Q can provide better rejection at 2.1 megahertz, thus a higher Q will not impact the audio band.
Let's look at the output filter block PCB layout. The ground returns from the LC filter should run between the output traces back to the ground pins of the IC to keep those loops short. And also, we have a lot of common-mode going on in the output stage filter, so when they return to the same path, we get a lot of cancellation due to the common-mode rejection of the system.
And we also would have to put the common-mode capacitors of the LC filter at the same point of the ground so that they also have a very good common-mode action and cancellation in the ground itself. We also allow for very many vias in the ground plane, and what this does is allow the high side FET currents to return back to the PVDD caps, which then return to ground.
We also have a 0.01 microfarad capacitor in the output stage, and those should be placed at the output connector and not close to the 1 microfarad LC filter capacitor. Again, this is an EMI capacitor that is necessary for a system, and it will be system dependent. So the 0.01 microfarad may be much smaller or a different value or a higher value than the 0.01 microfarad that we have on our EVM.
So what are the key market differentiators of the TPA6404-Q1? First of all, the 2.1 megahertz PWM switching is above the AM band, so we do not have any problems in the AM band for EMC. We also put proper location of ground pins on the power side of the device for good EMI mitigation. And the flow through design allows for the low current and high current areas to be separated, so in the conducted space of EMC, they do not contaminate each other.
We have other documentation on ti.com. We have a customer training series, webinar section called Measuring Class-D Audio Performance, and we have three technical blogs-- "Why should we switch to a Class-D Amp," "How switching above the AM Band eases automotive Class-D Amplifier EMC design," and "Forget the tiny hum craze. Have you heard about tiny inductors for automotive class d amplifiers?"

Details

Date:
April 10, 2018

In today’s automotive audio applications, space has become a premium. We will learn how to meet the space requirements and still meet automotive CISPR25 EMC requirements using the TPA6404-Q1 and surrounding components.